(63e) Influence of the Morphology of the Oxide Supports on Catalytic Performances of V/Sio2 and V/Mgo Catalysts in Methanol Partial Oxidation
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Monday, October 31, 2005 - 1:34pm to 1:50pm
The catalytic properties of supported vanadium-oxide catalysts are strongly influenced by the configuration of surface vanadia. The surface configuration is highly dependent on what metal oxide is used as the support; acidic supports lead to the formation of isolated vanadyl groups, polymeric vanadyl chains, and eventually over-layer crystals, e.g. V2O5, at high loadings, while basic supports can lead to mixed metal-oxide phases due to the strong interaction between the support and vanadia. While much work has been done comparing various supports (MgO, SiO2, Al2O3, etc.), few studies have compared different morphologies for the same oxide support. Aerogel techniques are capable of producing metal oxides with extremely high surface areas and unique surface chemistries. It is highly likely that the interaction between vanadium and aerogel-prepared metal oxides differs from the interaction with conventional metal oxides. For instance, it might be expected that aerogel-prepared metal oxides would be better able to disperse vanadyl groups due to enhanced surface area. Morphology changes for the same metal oxide might, therefore, lead to different vanadium surface structures and catalytic properties. This work studies how using an aerogel technique to prepare SiO2 and MgO affects the surface configuration and catalytic properties of vanadium in V/SiO2 and V/MgO catalysts. Both types of catalysts were prepared by the incipient wetness method and were characterized by BET, DRIFT-IR, Raman spectroscopy, XRD and H2-TPR. Chemical properties of the catalysts were probed by means of methanol partial oxidation.
The surface structures of V/SiO2 catalysts appeared to be identical on both types of silica supports at low vanadia loadings, with isolated three-legged (SiO)3V=O groups. At high loadings, various vanadia phases, including isolated VO4 and crystalline V2O5, may form on both aerogel and conventionally prepared silica supports. However, a small but clear Raman peak at ~1040 cm-1 was noted only for conventionally prepared catalysts; this is attributed to the formation of V2O5 ? nH2O gels resulting from the hydration of monovanadate groups and/or V2O5. These results suggest that there may be only isolated VO4 groups on aerogel-prepared silica for all weight loadings, while vanadium aggregates at high weight loadings on conventional silica.
All the V/SiO2 catalysts gave nearly the same product selectivities except when the weight loadings was low (0.5-2 weight%). For those weight loadings, the formaldehyde selectivities of conventionally prepared V/SiO2 were higher than those of aerogel-prepared V/SiO2 when the conversions were roughly the same. The differences in selectivity can be attributed to the support, rather than to differences in vanadium structure since isolated monovanadate species were expected on both samples. Silica is not selective to formaldehyde, instead producing carbon oxides and dimethyl ether. Aerogel-prepared silica possesses higher surface area and greater number of Brønsted acid sites (surface hydroxyls) than conventionally prepared silica, allowing greater influence on the catalytic behavior and less selectivity to formaldehyde at low loadings.
Catalytic activities and turnover frequencies for aerogel-prepared and conventionally prepared catalysts were nearly the same for weight loadings less than 3%. On the contrary, aerogel-prepared catalysts showed higher activities and turnover frequencies than conventionally prepared catalysts at 3 weight% and higher. The differences in turnover frequency were thought to be caused by differences in the vanadium phases present at high weight loadings on aerogel-prepared and conventionally prepared V/SiO2.
As evident from a Raman peak at 860 cm-1, magnesium orthovanadate was present for all of V/MgO catalysts with 20% weight loading and higher. In contrast, magnesium pyrovanadate was found only in the conventionally prepared catalysts with higher vanadium contents. No difference in selectivity was observed in methanol partial oxidation; however, aerogel-prepared V/MgO catalysts were more active than conventionally prepared samples at weight loadings higher than 15%. Again, the differences in catalytic results can be attributed to the higher surface area of the aerogel-prepared support, which allowed small domains of magnesium orthovanadate phases to exist, which were believed to be more active than magnesium orthovanadate or magnesium pyrovanadate clusters.
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